Lithium ion technology is the leading technology in the area of rechargeable battery accumulator systems for portable electronics. Lithium ion batteries are used as storage systems in mobile telephones, camcorders, laptops and for some time even in battery-powered tools. The next step to aim for is the use of lithium ion batteries in larger systems such as in automobiles or as stationary energy accumulators for renewable energy. Because of their high cell voltage, superior energy and power density as well as their low self-discharge, lithium ion batteries have a high potential for these applications. However, commercially available batteries do not fulfill the safety requirements for large systems. The thermal and chemical stability of the liquid electrolytes used plays an important part in this. Since heat arises with the discharge of lithium ion batteries, sufficient thermal stability of the electrolytes used is typically necessary for their use, especially for large systems with several hundred to a thousand kilowatt-hours of stored power.
Presently, lithium hexafluorophosphate (LiPF6) is used as a conductive salt in commercially available batteries. Lithium hexafluorophosphate has relatively high conductivity and is capable of forming a passivation layer, the so-called solid electrolyte interphase (SEI), on graphite electrodes. Lithium hexafluorophosphate, however, has considerable disadvantages because of its low thermal and chemical stability. It is known that LiPF6 reacts with traces of water and other protic compounds such as alcohol, which are not always completely avoidable in lithium batteries and occur, for example, in solvents in the ppm-area, and forms the toxic compounds POF3 and HF, which accelerate the disintegration of the spinels LixMn2O4 used as cathode materials as well as the degradation of the passivation layers both on the anode and the cathode. This reaction is accelerated by moderately elevated temperatures. This makes for a rapid loss of cell capacity that results in a shortened lifetime of the cell.
Thus, there are intensive efforts to develop alternative lithium salts that can replace LiPF6 as a conductive salt. Lithium salts developed in recent years are often complex boric and phosphoric anions with non-aromatic chelating agents like oxalate, for example lithium bis(oxalato)borate (LiBOB) disclosed in DE 198 29 030 C1. However, there is a disadvantage in that bis(oxalato)borate has only slight solubility in carbonates used as solvents in electrolytes. LiBOB-based electrolytes also have lower conductivity and higher viscosity in comparison with LiPF6. In particular, bis(oxalato)borate electrolytes have only slight conductivity at low temperatures. Moreover, the production of bis(oxalato)borate of sufficient purity is expensive, since the contamination with oxalate and carboxylate at elevated temperatures leads to the escape of gases from the cells. A further disadvantage of the use of lithium bis(oxalato)borate is that an overly strong SEI is formed by which cell resistance is increased.
In spite of a multitude of salts and solvents, still no suitable replacement for LiPF6 as a conductive salt in carbonate mixtures has been found. Thus, there is a need of alternative lithium salts.